Detect language on server side instead of JS

This MR fixes #3204 (closed) and preserves behavior from #2899 (closed) by computing the user's preferred language on the server side. Logic is still the same:

• first, consider llnglanguage
• then, Accept-Language
• finally, fallback to "languages" / english

Moving everything to server-side avoids having duplicate logic (in two different programming languages), which could have been a future source of bugs.